1. Field of the Invention
This invention relates to a process for the preparation of an indigo compound. More specifically, it relates to a process for the preparation of an indigo compound by reacting an indole compound having no substituent at the 2- and 3-positions with a percarboxylic acid in the presence of a specific solvent.
2. Description of the Related Art
Indigo compounds are important compounds that are useful as dyes. The presently employed industrial processes for the preparation of indigo comprise forming an N-phenylglycine salt from aniline and chloroacetic acid, or from aniline, cyanic acid and formaldehyde, converting this salt into an indoxyl compound by alkali fusion at elevated temperature, and then oxidizing this compound with air. However, these processes are not only complicated ones involving many steps, but also require the use of large amounts of potassium hydroxide and sodium hydroxide. Moreover the recovery and reuse of used potassium hydroxide and sodium hydroxide has the disadvantage of consuming much energy and requiring special equipment. Therefore, conversion to a simpler process has been desired.
Meanwhile, there is a report that indole have been reacted with perbenzoic acid, which is a percarboxylic acid, in chloroform as a solvent by allowing the reaction mixture to stand in a refrigerator overnight (Justus Liebigs Annalen der Chimie, Vol. 558, pp. 91-98, 1947). According to the article, it has been reported that o-formaminobenzaldehyde was produced together with a variety of other products, and a very small amount of indigo was also formed at the same time. Moreover, there is another report that peracetic acid, which is a percarboxylic acid, was produced from hydrogen peroxide and acetic acid in the reaction system and reacted with indole in acetic acid as a solvent (Bull. Agr. Chem. Soc. Japan, Vol. 20, pp. 80-83, 1956). According to the report, it has been reported that 2,2-diindyl-.psi.-indoxyl, which is a trimer of indole skeleton, was produced as a main product and in addition a very small amount of indigo was formed as a by-product. However, each of these reports was concerned with a brief investigation on the reactivity of indole, main products were o-formaminobenzaldehyde and 2,2-diindyl-.psi.-indoxyl, respectively, and indigo, which is the desired product in the present invention, was nothing but a by-product formed in very low yield.
In application Ser. No. 07/314,021, filed Feb. 23, 1989, filed by us and another, the conversion of an indigo compound lacking 2- and 3-position substituents to an indigo compound is achieved with an organic hydroperoxide; in our Ser. No. 07/324,503, it is achieved with hydrogen peroxide; in our Ser. No. 07/343,459, it is achieved with an organic hydroperoxide in the presence of a carboxyl compound or a boric acid ester compound as an additive.